For example, semiconductor wafers such as silicon wafers are usually subjected to chamfering of their peripheries mainly to prevent their edges from chipping or to prevent crowns during epitaxial growth. The chamfering is performed by grinding with a diamond grinding wheel; however, distorted layers are apt to remain after grinding. The remaining distorted layers lead to crystal defects in some cases when heat treatment is repeated in a device process.
For that reason, the distorted layers are normally removed by etching. Etched surfaces, however, tend to develop undulated or scale-like irregularities which tend to retain soil. The soil spreads over an entire wafer in the device process, contributing greatly to deterioration of characteristics.
In recent years, a technique for smoothing chamfered edges of wafers by mirror polishing has been established as a technique completely different from that for polishing the surfaces of wafers. The applicants of the present invention have already proposed a technique for polishing edges as disclosed, for example, in Japanese Unexamined Patent Publication No. 1-71656. According to the polishing technique, a wafer having a chamfered edge on its outer periphery is rotated, and the outer periphery edge is pressed against a working plane of the outer periphery of a rotating polishing drum thereby to polish the outer periphery edge. This method enables wafer edges to be polished easily and reliably, and solves all the problems caused by the chamfering mentioned above.
However, this type of polishing apparatus is designed to perform polishing by bringing wafers into point contact with polishing drums; hence, machining efficiency is not always high, taking a considerable time for the machining. In these days, therefore, contrivance has been added. An example of such contrivance is increasing the diameter of a polishing drum to maximize the length of contact with a wafer so as to shorten the machining time.
However, a method in which a round wafer is brought into circumferential contact with a cylindrical working plane is disadvantageous in that there is limitation in extending the contact length, so that reducing the machining time is accordingly limited. In addition, increasing the diameter of the polishing drum means a larger space required for installation, inevitably resulting in an increased size of the apparatus including the drum. Especially because demands for larger wafers having diameters ranging from 30 cm to 40 cm are expected in the future, which means larger spaces occupied by the wafers, the polishing apparatuses would undesirably be even larger.